U.S. Pat. No. 5,737,035 dated Apr. 7, 1998, shows a tuner circuit. The front end of such a tuner requires a broadband, highly linear Variable-gain Low Noise Amplifier (VLNA). The VLNA's input comes from either an antenna for wireless broadcasts or from a coaxial cable for cable transmission. The output of the VLNA supplies the input of the first up-converting mixer. The noise figure specification for the VLNA is highly critical, and has the highest impact on the overall noise figure of the system. Non-linearities of the amplifier also have a large effect on the proper operation of the tuner.
LNA's are typically used to meet cellular requirements, which are narrow-band. However, a television tuner must receive carriers from 50 MHZ to over 860 MHz. A narrow bandwidth system also has less stringent linearity specifications because fewer intermodulation distortion products fall in-band. Finally, because the incoming signal power to a tuner can vary by many orders of magnitude, there is a need for an LNA whose gain is continuously-variable. The gain variability function adds noise, distortion, and complexity to the LNA.
Accordingly, there exists a need in the art for a low noise amplifier which can handle a wide frequency range as well as a wide amplification range.
A further need exists in the art for such a circuit which can be constructed on a single substrate and which will not be effected by, nor produce, noise injection.